The invention relates to a pressure control valve that serves to generate a variable working pressure for actuating a device such as the start-up clutch of a motor vehicle, where the variation of the working pressure is effected by a controlling pressure, e.g., a pilot pressure, while the supply pressure delivered to the pressure control valve by a pressure source, for example a pump, remains constant.
The functional dependency between the pilot pressure and the working pressure is subject to a diversity of requirements. For example, in the engagement phase of a start-up clutch, a change in the pilot pressure should at first cause only a small change in the working pressure in order to allow a precise control of the working pressure. However, with a continued increase in the pilot pressure, the working pressure should increase at a steeper rate to ensure that the clutch is securely engaged and works in a slip-free or at least substantially slip-free manner.
This kind of a progressive pressure control characteristic can be realized, e.g., with the known pressure control valve according to DE 198 20 389 A1, which has a control piston coaxially surrounded by an outer piston with at least three radial openings. The pressure control valve has connector conduits, one of which introduces the working pressure to the control piston in such a manner that the control piston moves in opposition to the combined forces of a spring and the pilot pressure. A further conduit performs the function of applying the pilot pressure to the outer piston. When a certain threshold value of the pilot pressure is exceeded, the outer piston will move in opposition to the force of another spring, whereby the aforementioned spring force that opposes the movement of the control piston is increased. As a result, the gradient slope of the working pressure in function of the pilot pressure is increased, so that the pressure control valve has a correspondingly non-linear characteristic.
The pressure control valve of the foregoing description suffers from the drawback of a relatively complex design and requires a comparatively complicated manufacturing process.
It is therefore the object of the present invention to provide a pressure control valve of a less complicated design, in which a desired functional relationship between pilot pressure and working pressure can be achieved through an appropriated choice of dimensions, in particular for providing the pressure control valve with a progressive characteristic where the working pressure as a function of the pilot pressure rises at a steeper gradient in that part of the characteristic where the pilot pressure is high than in the part where the pilot pressure is low.
According to the invention, the foregoing objective is met by a pressure control valve with a working-pressure chamber and a pilot-pressure chamber. The working-pressure chamber receives a supply pressure from a pressure source by way of a supply-pressure port and a throttle passage and delivers a working pressure to a pressure-actuated apparatus by way of a working-pressure port. The pilot-pressure chamber receives a pilot pressure from a pilot-pressure control device. The working-pressure chamber is axially confined between two piston elements of a control piston, and the pilot-pressure chamber is axially confined between one of the control-piston elements and an additional piston. In the presence of a constant supply pressure, the working pressure generated at the working-pressure port varies as a function of the pilot pressure. The additional piston takes a position dependent on a momentarily existing difference between the working pressure and the pilot pressure and, through an elastic force-coupler device, applies a position-dependent force to the control piston. In addition, the valve has one or more drain ports.
Preferably, the aforementioned force-coupler device is designed as a coupler spring arranged in such a manner that one end of the spring acts against the additional piston while the other end of the spring acts against the control piston.
In a further developed embodiment of the invention, the additional piston is elastically biased into a rest position and connected to the working pressure in such a way that the additional piston will leave the rest position only when the working pressure exceeds the pilot pressure by a predetermined amount.
In an embodiment that includes the aforementioned coupler spring as well as the elastic biasing of the additional piston, it is advantageous if the coupler spring is in a force-free state when the additional piston is in the rest position.
The elastic biasing of the additional piston can be advantageously performed by a biasing spring.
In preferred embodiments of the invention, the housing of the pressure control valve has a cylindrical interior space divided into three cylinder-bore sections of different respective diameters arranged in order of diameter size from smallest to largest. The first piston element of the control piston runs inside the first and narrowest cylinder-bore section, while the second piston element runs in the second, i.e. intermediate, cylinder-bore section and the additional piston runs in the third, i.e. widest, cylinder-bore section. As described above, the working-pressure chamber is confined between the first and second piston elements, and the pilot-pressure chamber is confined between the second piston element and the additional piston.
With the arrangement of three cylinder-bore sections as just described, it is advantageous if the drain port is located in the intermediate cylinder-bore section.
In design arrangements where the pressure control valve has three cylinder-bore sections, it is further advantageous if the supply-pressure port is arranged to enter the valve in the area of the first cylinder-bore section at a place occupied by the first piston element. The aforementioned throttle passage from the supply-pressure port to the working-pressure chamber can be designed as a narrow gap between the circumference of the first piston element and the inside cylinder wall of the first cylinder-bore section.
In a further preferred embodiment, the pressure control valve according to the invention has a dead-end chamber on the side of the first piston element that faces away from the working-pressure chamber. The dead-end chamber is open to an additional drain port. The aforementioned narrow gap between the first piston element and the housing now throttles the fluid flow from the supply-pressure port in both directions, i.e., into the working-pressure chamber as well as into the dead-end chamber.
The pressure control valve according to the present invention is distinguished by a comparatively simple design and by its adaptability to diverse requirements. By appropriate dimensioning of the two springs and of the effective pressure-bearing surfaces of the pistons, it is possible to achieve a desired valve characteristic, i.e., a desired functional relationship between the pilot pressure and the working pressure.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.